When a coil is rotated in a magnetic field, the coil cuts the lines of the magnetic field, so an emf is induced in the coil. If the coil is part of a complete circuit, the work done in moving the coil in the magnetic field is transferred to electrical energy (i.e. the kinetic energy of the electrons).The current through the coil causes a reaction force on the coil due to the motor effect. Work must therefore be done to keep the coil spinning.